The present invention relates to a fuel injection device for an internal combustion engine such as an automobile engine or the like and, more particularly, to the construction of an ultrasonic fuel injection nozzle provided with a function for ultrasonically atomizing liquid fuel to be injected into a gasoline engine.
Development of internal combustion engines has been directed towards increasing their performance and efficiency in relation to fuel economy and combustion efficiency and with due consideration to the regulation of exhaust fumes. For instance, in order to obtain an optimum air-fuel ratio in automobile engines, the tendency to adopt fuel injection methods instead of carburetor methods has been observed. In this connection new fuel vaporizers with electronic controls based on the high accuracy of air flow measurements have been developed and further adapted to a total computer control system for ignition timing, knocking, EGR (Exhaust Gas Recirculation) and other engine dynamic parameters. The improvement of engines has advanced through step-by-step improvement in each of the engine's dynamic characteristics.
A fuel injection nozzle is a device that injects liquid fuel in quantities adapted to the measured value of the suction air flow. Quantities of fuel are proportional to the valve opening duration i.e. fuel is discharged with a constant pressure through a fixed valve opening determined by a control signal during the valve opening duration defined by ON-OFF control signals. Furthermore, for increasing efficiency of the fuel's burning, attempts have been made to improve the shape of an injection port and/or a needle of the injection valve in order to produce a shearing force to the fuel being injected. Recently, fuel injection nozzles with ultrasonic atomization have also been proposed. An example of such a nozzle is described in the Japanese laid open patent publication No. 70656/87. A fuel injection nozzle disclosed in the above-mentioned laid open patent publication, basically, composed of a fixed needle and a vibrator which, being provided with a valve functioning in association with said needle, may shut off and spout a flow of fuel into the needle and then atomize the injected fuel by the action of ultrasonic vibrations. The needle is a cylinder which has a top flange with a fuel feeding opening to be connected with a fuel-feeding means, a fuel passage with a filter inserted therein for cleaning the fuel, an annular through-hole communicating with the top end of the fuel passage and with the outside of the needle, and a needle valve integrally formed at the tip end. The vibrator, in which the needle is loosely fitted with the preferable clearance of about 20 microns, has an upper side flange whereto an ultrasonic vibration generating means, being composed of a ring-shaped piezoelectric element, is fixed by the use of a nut, and has at its lower part a center injection hole, an outwardly enlarged, multi-stepped opening and a valve seat abutting onto the needle valve. The housing, enclosing a vibrator with a needle loosely inserted therein is screwed to the threaded portion of the flange of the needle. A spring means for pressing the vibrator to the needle to make the needle normally closed is interposed between the lower surface of the flange of the vibrator and the shoulder part of the housing near the injection port. A means for generating an attracting force, which is composed of ring-shaped laminated piezoelectric elements which are insulated from the needle by a kind of insulation, is placed between the upper surface of the nut of the vibrator and the lower surface of the flange. When a pulse signal of voltage is applied to the attracting force generating means, the vibrator, against the force of the spring means, moves downward to open the valve. When the valve is opened, the fuel, having flowed into the vortex chamber from the passage through the annular through-hole, spouts out through the injection port and flows laminatedly through the stepped-hole and is atomized by virtue of the vibrations produced by the vibrator during an ultrasonic signal being applied to the ultrasonic vibration generating means.
The application of an ultrasonic signal to the ultrasonic vibration generating means may be synchronized or done at different times with relation to the application of pulsating voltage to the attracting force generator. The above-mentioned "prior art" has some problems as shown by the following:
The first problem is with the laminated piezoelectric element used in the attraction force generating means since it has a higher response in comparison with the generally used electromagnetic means such as an exiting coil for generating an attraction force. However, since the above-mentioned vibrating system is related to the mass of the vibrator and the elasticity of the spring means, the basic condition for improving the response speed of the system is to reduce the mass of the vibrator. However, the vibrator is a large-sized cylinder having a needle loosely inserted therein and a stepped portion that brings a result contrary to the above-mentioned purpose.
The second problem is that, the vibrator may be driven synchronously or with a certain time-difference in relation to the ultrasonic vibration generation, however, since the vibrator, if being of large mass, may have a delay in motion, it is rather difficult to practically synchronize the vibrator's drive with the ultrasonic vibration generator's drive. Consequently, in case of low speed operation of the device it becomes impossible to ultrasonically drive the vibrator in time to effectively atomize the fuel.
The third problem relates to the method for adjusting the force of the pressure of the spring means. To improve the response speed of the ultrasonic vibrating system it is also necessary to adjust the force of the spring means to an optimum value. In a conventional device, a means for adjusting the spring's force is a threaded portion by which the spring's force cannot correctly be adjusted.
The fourth problem is that a displacement of the vibrator by the action of the attraction force generating means is decided by the voltage being applied to the piezoelectric element. Since the piezoelectric constant may vary in accordance with the temperature of the element, the above-mentioned method cannot assure the correct opening of the valve i.e. the flow curve is correctly proportional to the width of the pulse signal applied to the element.
Furthermore in the Japanese laid open patent publication No. 222552/85 a method is disclosed where liquid fuel is pulverized into fine particles by forcing the fuel through a vibrator being driven by ultrasonic waves. Ultrasonic atomization of liquid fuel may be done in such a way that liquid fuel is periodically fed into an atomizing chamber with the continuous excitation of ultrasonic vibrators or with synchronous periodical excitation of ultrasonic vibrators. For example, in the case of using an ultrasonic atomizing unit as shown in the Japanese laid open patent publication No. 222552/85 for injecting atomized fuel into an internal engine, it is shown that fuel is periodically fed into an atomizing chamber wherein vibrations are continuous. Furthermore, in the Japanese laid open patent publication No. 138557/86, an electromagnetic ultrasonic injection nozzle is proposed which is based upon that shown in the Japanese laid open patent publication No. 222552/85. The above-mentioned electromagnetic ultrasonic injection nozzle comprises an ultrasonic generator, a slender vibrator connected at one end to said ultrasonic generator and having an edge portion at its other end, said vibrator being loosely inserted into a housing, a hollow needle valve having a core integrally fixed to its upper end and being slidably fitted on said vibrator so as to be positioned near the edge portion of the vibrator, a fuel passage for supplying liquid into the edge portion, a spring means pressing said hollow needle valve to normally keep said passage closed, an electromagnetic means for exerting the core to move the hollow needle valve against the force of the spring means and thereby to open the passage, and a stopper abutting an annular slot of the hollow needle valve and defining the limits of the movement of said hollow needle valve by an axial clearance between the stopper and said annular slot of the needle valve. The quantity of liquid fuel flowing into the edge portion through the open passage is proportional to the duration of time for keeping the electromagnetic means energized. The vibrator is fixed at one end with a mounting plate which serves a node of the vibration system and has the edge portion at its other end to create ultrasonic vibrations. Liquid fuel is subjected to atomization by ultrasonic vibrations and directed to a combustion chamber.
In the above-mentioned prior art, the fixed quantity of liquid fuel, which is proportional to the energized duration of the electromagnetic means, may be introduced into the edge portion of the vibrator by maintaining a constant pressure of fuel to be supplied through the passage. Since the movement (mass) of the hollow needle valve together with the spring force of the spring means and the electromagnetic force of the electromagnetic means form the secondary vibration system, to realize a quick-response of the movement of the hollow needle valve it is necessary to adjust the pressure of the spring means. However, the prior art does not show any adjusting means. For the practical use of the ultrasonic fuel injection nozzle it is necessary to provide a means to adjust the spring force of the spring means.